lecture 7 (feb 5).pdf

WEST VIRGINIA UNIVERSITY Physics

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Today’s lecture

The laws of motion: • What is a force?

• Newton’s 1st law • Newton’s 2nd law


Motivation

• We have described motion in 1D and 2D, but what causes motion?

• In the late 1600s, Sir Isaac Newton formulated three laws governing moving objects, which we call Newton’s laws of motion, after complex deduction from experimental results.

• The laws are simply stated, but incredibly profound.


What is a force?


What is a force? - Newton’s 2nd lawThe acceleration of an object is directly proportional to the net force

acting on it and inversely proportional to its mass.

• This is one of the most important equations in physics.

• It means that every force on an object results in an acceleration of the object, i.e. a change of its velocity.

Note 1: There can be multiple forces acting on one object, that compensate each other. Then, there is no acceleration.

Note 2: An acceleration does not have to cause a change of the velocity’s magnitude, but could change its direction only (circular motion is an accelerated motion).

Unit: 1 N = 1 kg m/s2


Superposition of forces

• Forces are vectors and must be added using vector addition.

• This can be done graphically (parallelogram) or algebraically.

• The vector sum of all forces acting on an object is called the resultant force or the net force.


Example: Adding 3 Forces

F1 = 250 N, θ1 = 127°

F2 = 50 N, θ2 = 0°

F3 = 120 N, θ3 = 270°

Rx = F1x + F2x + F3x

Ry = F1y + F2y + F3y


Newton’s first law

…is actually a special case of Newton’s second law. If no force acts on an object or all applied forces compensate:

If there is no effective force, the object’s velocity does not change.

An object moves with a velocity that is constant in magnitude and direction unless a non-zero net force acts on it.

Let’s look at some consequences of Newton’s first law…..


Newton’s first law


Consequences of Newton’s first law

In space there is no friction and (except weak gravity) no forces.

Thus, once accelerated in one direction an object will continue moving in this direction forever.

When you ride a car, you and the car are moving at the same speed.

If you do not use the seat belts and the car breaks, a force will act on the car to decelerate it, but it will not act on you.

Thus, you continue to move at the original speed according to Newton’s 1st law.


Inertia, mass, and weight

Inertia is the tendency of an object to continue in its original state of motion.

Mass is a measure of the object’s resistance to changes in its motion due to a force.

The higher the mass, the smaller the acceleration for a given force.

Weight is the magnitude of the gravitational force acting on an object of mass, m.

The weight depends on the gravitational constant, g, that is different for different planets.


Example: Application of Newton’s Laws

An 80 kg movie stuntman jumps from a window of a building situated 30 m above a catching net. Assuming air resistance exerts a 100-N force on the stuntman as he falls, determine his velocity just before he hits the net.


Summary• A force is the product of an object’s mass and acceleration. Forces are the reason why

objects change their velocity. Newton’s second law:

• Forces are vector quantities, since they have magnitude and direction.

• An effective force changes an object’s velocity (magnitude and/or direction). A circular motion is an accelerated motion.

• Newton’s first law is a special case of Newton’s second law (F = 0):

An object moves with a velocity that is constant in magnitude and direction unless a non-zero net force acts on it.

• Inertia is the tendency of an object to continue in its original state of motion. Mass is a measure of the object’s resistance to changes in its motion due to a force. Weight is the magnitude of the gravitational force acting on an object of mass, m:

Unit: 1 N = 1 kg m/s2

lecture 7 (feb 5).pdf

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